Aug 05, 2013 |
A layer of tiny grains can slow sound waves
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(Nanowerk News) In some ways, granular material — such as a pile of sand — can behave much like a crystal, with its close-packed grains mimicking the precise, orderly arrangement of crystalline atoms. Now researchers at MIT have pushed that similarity to a new limit, creating two-dimensional arrays of micrograins that can funnel acoustic waves, much as specially designed crystals can control the passage of light or other waves.
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The researchers say the findings could lead to a new way of controlling frequencies in electronic devices such as cellphones, but with components that are only a fraction the size of those currently used for that function. On a larger scale, it could lead to new types of blast-shielding material for use in combat or by public-safety personnel or equipment.
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A paper on the research appears in the journal Physical Review Letters ("Interaction of a Contact Resonance of Microspheres with Surface Acoustic Waves"), written by Nicholas Fang, the Brit and Alex d’Arbeloff Career Development Associate Professor in Engineering Design; Nicholas Boechler, a former MIT postdoc now at the University of Washington; and four co-authors.
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Photograph of a two-dimensional array of microspheres adhered to a substrate. (Photo courtesy of Tian Gan)
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Research on the properties of granular materials — collections of small grains, such as sand or tiny glass beads — has become “a rich and rapidly developing field,” the researchers write. But most such research has focused on the properties of sand-sized particles, about a millimeter across, Fang says. The new work is the first to examine the very different properties of particles that are about one-thousandth that size, or one micrometer across, whose properties were expected to be “qualitatively different.”
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In their experiments, the team used a single layer of microspheres to guide and slow sound waves (known as surface acoustic waves, or SAWs) traveling across a surface, Fang says. The researchers used ideas they had previously applied in research on controlling light waves, he says, which involved the use of photonic crystals.
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SAWs are widely used in electronic devices such as cellphones, Fang says, “like clocks that give a single frequency signal … to synchronize different chips or parts of a chip.” But with the new system, “we can shrink the device size” needed for processing SAWs, he says. Present-day oscillators for SAWs are relatively bulky, Fang says, but the use of a 2-D granular material to guide and slow the waves could allow such devices to be only one-sixth their present size, he estimates.
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What’s more, the 2-D nature of this system could allow it to be fabricated right on a chip, along with the necessary control circuits and other components. Today’s oscillators, by contrast, are typically separate devices placed next to the chip array that controls them, Fang says — so in cases where small size is important, the new work has the potential to allow for even smaller electronic devices.
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The system could potentially also be used to develop new kinds of sensors, such as microbalances capable of measuring tiny changes in weight, he says.
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The same principle could also lead to a new kind of blast-shielding material, Fang suggests. If acoustic waves — such as the intense shock waves from an explosion — hit the two-dimensional material at a right angle, much of their energy can be converted to surface waves that travel sideways out of the material. A sandwich of many layers of such material might provide substantial protection from a blast in a lightweight, wearable form, though such applications will likely require substantial further research, Fang says.
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John Page, a professor of physics and astronomy at the University of Manitoba, says this is “a high-quality piece of research. … I am sure that their findings will be widely accepted.”
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